Section 37. No producer of milk shall be liable to prosecution for the reason that the milk produced by him is not of good standard quality unless such milk was taken upon his premises or while in his possession or under his control by an inspector of milk, collector of samples of milk or agent of the division of dairying and animal husbandry of the department of food and agriculture or of the department of public health, and a sealed sample thereof was given to such producer upon demand, nor unless he shall fail to bring the milk produced by him to the legal standard for milk solids and milk fat within twenty days after written notice that it is below said standard has been sent to him by the officer taking said sample. At any time after such period of twenty days has elapsed, any such officer may take a second sample, and if it is found to be below the legal standard for milk solids and milk fat prosecution may follow.